The present invention relates to a planar light source for a backlight which is, for example, provided for illuminating a liquid crystal display (LCD) panel from behind, and more particularly to a light source provided with a light guide plate, a light emitting source such as a light emitting diode (LED) disposed at a side of the light guide plate, and preferably, a light direction limiting means such as a diffusion panel disposed above the light guide plate. In such a light source, the direction of light emitted from the light emitting source is changed so that a luminous flux exits the light guide plate from the upper surface thereof, and preferably, the direction is further adjusted by the light direction limiting means.
There is known a backlight unit comprising a planar light source for illuminating such a display as an LCD provided in a mobile terminal device and laptop computer. Japanese Patent Application No. 2002-146589 (Japanese Patent Application Laid-Open No. 2003-337333) discloses such a backlight unit in FIG. 17, which is shown in FIGS. 12a and 12b. FIG. 12a is a perspective view showing the backlight unit and FIG. 12b is a sectional view of the backlight unit.
A backlight unit 120 comprises a pair of LEDs (light emitting diodes) 102, a light guide plate 101, a diffusion panel 103, a Py prism sheet 104, a Px prism sheet 105, a reflection plate 106, and a transmissive or semi-transmissive LCD 107. The plates 101, 103, sheets 104, 105 are stacked.
The light guide plate 101 is made of a transparent plastic and has an upper surface 101a, lower surface 101b and front side 101c. The LEDs 102 are mounted on an LED substrate 102b and disposed opposite the front side 101c. The diffusion panel 103 is disposed above the upper surface 101a of the light guide plate 101 and the reflection plate 106 is disposed below the lower surface 101b. 
The light emitted from the LEDs 102 enters the light guide plate 101 from the front side 101c. The entered light is repeatedly reflected by the upper and lower surfaces 101a and 101b and advances through the light guide plate 101. The lower surface 101b has a fine prism surface so that, in accordance with the principles of the reflection and refraction of light, a part of the light is reflected toward the upper surface 101a while the rest is refracted and enters the reflection plate 106. On the surface of the reflection plate 106, the light is reflected so as to return to the light guide plate 101. The reflection plate 106 thus serves to increase the light utility efficiency.
The direction of the discharged light is arranged in a small range by the diffusion panel 103, and further arranged by the prism sheets 104 and 105 in the Y and X-directions, and finally arranged in the Z-direction. The light arranged in the Z-direction illuminates the LCD 107. Hence the light is transmitted through the liquid crystal under an optimum condition, thereby enabling a clear display of high S/N ratio.
However, there is a following problem in the conventional planar light source. Namely, since the light is reflected in various directions at the lower surface 101b, not a small number of light beams enter the upper surface 101a at an angle approximate to the critical angle as shown in FIG. 12c. Such a light beam refracts at an angle approximate to 90 degrees to the normal, that is, almost horizontally. In such a case, the light beam does not enter the diffusion panel 103, or even if it does, the incident angle is so large that it is difficult to efficiently change the direction of the light beam and render it to exit from the diffusing plate 103 and enter into the Py prism sheet 104. Hence it is difficult to efficiently change the light entering the light guide plate 101 from the LED 102 into a sufficiently bright illuminating light.
In order to resolve the problem, there is proposed a light source shown in FIGS. 13a to 13c where the light guide plate 101 has an anisotropic diffusing surface 101h having a plurality of longitudinal hairlines or holograms provided to cause anisotropic diffusion. In the illustrated example, a plurality of hairline prisms are formed on the lower surface 101b. Other constructions of the light source are the same as that of the backlight unit 120 of FIG. 12.
The operation of the device is based on the well-known principle described on page 5 of U.S. Pat. No. 6,347,873, for example. More particularly, since the anisotropic diffusion surface 101h is formed on the upper surface 101a of the light guide plate 101a, the incident angle of the light beam applied thereto becomes a desired angle larger than 90 degrees, far from the horizontal direction. Hence, a tapered diffracted light beam φ01 is generated even in the case where the incident angle of the incident light beam is approximate to the critical angle. Thus the incident angle of the light beams entering the diffusion panel 103 is increased so that the light utility efficiency is improved and the brightness of the illuminating light is increased.
Meanwhile, the diffused light beam φ01 is an anisotropically diffused light beam having a longer axis in the longitudinal direction of the anisotropic diffusion surface 101h than in the lateral direction as shown by the section thereof in FIG. 13c. The diffusion surface 101h is thus adapted to have a large diffusion characteristic in the longitudinal direction thereof. The reason the anisotropic diffusion surface is so adapted is that although the diffusion caused by each of the numerous grooves overlaps each other in the lateral direction of the surface 101h so that the light is sufficiently laterally diffused, in the longitudinal direction, it is necessary to render the diffusion at each groove wider in the longitudinal direction than in the lateral direction.
However, there is another problem even in the thus improved planar light source when examined more closely. Referring to FIGS. 13b and 13d which are enlarged diagrams showing the part adjacent the front side 101c of the light guide plate 101, a light beam s01 reflected at the lower surface 101b and a light s02 reflected at the upper surface 101a are examined. The light beam s01 is refracted at the prism of the lower surface 101b and further reflected by the reflection plate 106 so as to again reach the lower surface 101b. The beam is refracted and enters the diffusion surface 101h of the upper surface 101a at an incident angle approximate to the critical angle but smaller. Accordingly, the beam exits the diffusion surface 101h as a diffused light beam φ01 at an angle within a predetermined range without reflecting. Since the upper surface 101a is not smooth, the beam does not exit horizontally.
On the other hand, supposing the initial incident angle of the light beam s02 is larger than the critical angle, the beam enters the diffusion surface 101h. Contrary to the case where the upper surface 101a is smooth so that the incident light is 100 percent totally reflected, due to the diffusion, a quite a large part of the light exits as a secondary diffused light beam φs as shown by the dotted lines in FIG. 13b. The rest of the light beam is reflected and permeated through the lower surface 101b, reflected at the reflection plate 106, refracted at, the lower surface 101b and reaches the upper diffusion surface 101h. Due to the operation of the prism at the lower surface 101b, the incident angle at the diffusion surface 101h is gradually reduced each time the beam is reflected. In the example shown in the figure, in the light beam s02, after going through the reflection twice, the incident angle at the diffusion surface 101h becomes smaller than the critical angle so that only a diffused light beam φ02 is emitted. Since the intensity of the diffused light beam φ02 decreases by that of the secondary light beam φs every time the beam is reflected, the intensity of the diffused light beam φ02 becomes smaller than that of the diffused light beam φ01.
In addition, as shown in FIG. 13d, the diffused light beams φ01 and φ02 are able to enter the diffusion panel 103 in ranges having widths W01 and W02, respectively. Since the distance between the light guide plates 101 and the diffusion panel 103 cannot be increased, it is impossible to enlarge the widths W01 and W02.
More particularly, as explained with regard to the light beams φ01 and φ02, regarding a single internal light beam, although diffused at the diffusing surface 101 h, the widths W01 or W02 of the range through which thee beam reaches the diffusion panel 103 is small. In order to increase the range, it is necessary to let the light beams enter the diffusion surface 101h at various incident angles so that the position of exit of the beams φ01 and φ02 vary. As the position is moved toward the right in the drawing, the number of reflections at the diffusion surface 101h increases, which results in decrease in intensity of the light beam. Hence the luminance of the output light is decreased as the distance from the front surface 101c increases toward the right side of the light guide plate in FIG. 13b. Consequently, the intensity of incident light entering the diffusion panel 103 also decreases at the right side so that the distribution of luminance becomes uneven. The light transmitted through the diffusion panel 103 is adjusted in X-, Y-, and Z-directions by the Py prim sheet 104 and Px prism sheet 105 and becomes the illuminating light. However, the illuminating light also becomes uneven in luminance.
Namely, in the planar light source provided with the anisotropic diffusion surface on the upper surface of the light guide plate, the intensity of light which exits the diffusion surface becomes lower toward the right, that is, as the distance from the LED increases. As a result, the luminance of the light exiting the light guide plate and entering an optical path adjusting means such as the diffusion panel becomes uneven depending on the position. Such a tendency is retained even in light entering the LCD 107 after passing through the optical path adjusting means so that the quality of illumination is decreased.